A low-cost and compact optical scanning apparatus having the following configuration has been developed (refer to PTL 1). A plurality of light beams are made incident on different deflection surfaces of a light deflector at an angle in a sub-scanning cross section. Four photosensitive drums are arranged in parallel. For each of the two outer drums, the optical path is bent by a single mirror. For each of the two inner drums, the optical path is bent by two mirrors. In addition, a compact optical scanning apparatuses including a simplified optical unit using a prism as a beam splitting unit for removing restriction of the layout of the optical scanning apparatus has been developed (refer to PTL 2).